Compositions which crosslink to give elastomers and are said to have a low compression set have already been described.
DE 41 00 217 (Degussa AG) discloses that thiocyanatopropyltrialkoxysilanes can bring about an improvement of the compression set in molding compositions which can be vulcanized using peroxides together with heat. The same disclosure describes that the organosilicon compounds can be used either separately or in combination with a filler. DE 33 19 251 (Degussa AG) discloses the use of isothiuronium compounds in place of thiourea derivatives for vulcanizable halogen rubber mixtures. The resultant vulcanizates were found to have better strength, tensile modulus, abrasion and compression set values than comparable halogen rubber mixtures without isothiuronium compounds. In EP 0 278 157 (Shin Etsu Chemical Co., Ltd.), thiocarbamyl-functional organosilanes are added, in order to improve the compression set, to rubbers which are vulcanizable using organic sulfur compounds or using organic peroxides.
Other proposals for improving the compression set are based on the use of additives such as, barium peroxide, barium oxide or magnesium oxide (U.S. Pat. No. 3,261,801, U.S. Pat. No. 3,468,838 and U.S. Pat. No. 3,865,778), and the use of polycarbodiimidepolysiloxane copolymers (EP 0 010 708) which give strongly colored products.
The above mentioned methods for improving the compression set are relevant to rubbers which may be crosslinked to give elastomers by using organic sulfur compounds or organic peroxides.
By contrast, organopolysiloxane compositions which crosslink via an addition reaction, preferably via a hydrosilylation reaction of vinyl-containing siloxanes, are catalyzed by a number of platinum compounds. For improving the compression set in addition-crosslinking systems, iron-manganese spinels of the empirical formula Fe.sub.y Mn.sub.z O.sub.4 (Wacker Silicones: EP 582 299), triazole compounds (Shin Etsu Chemical Co., Ltd.: U.S. Pat. No. 510,49,19), vinyl-specific organic peroxides &gt;0.2% by weight (Dow Corning Co.: EP 0 377 185), and transition metal chalcogenides (Bayer AG:EP 0 455 078) have been described.
The above mentioned compositions have the disadvantage that they do not provide addition-crosslinking compositions which are transparent and have a low compression set.